Fractal column offset tension leg platform (COTLP)

ABSTRACT

A Fractal Column Offset Tension Leg Platform (COTLP) consists of a semi-submersible hull type, tension moored to the seabed. The Fractal COTLP advances the state of technology by utilizing fractal arrangements of repeated, similar structural units, consisting of continuous pontoons and intermittent columns, to provide better construction, operation, stability, payload and service life advantages over existing Tension Leg Platform (TLP) types. Varying, fractal-like geometric arrangements also provide for sizing and configuration advantages and the capability to work in greater water depths than existing TLP technology. Furthermore, fractal arrangements and inter-column spacing can also be optimized to minimize platform hydrodynamic response to waves, thereby reducing mooring and hydrodynamic loads and consequently achieving an optimal and safer platform structure with a longer fatigue life.

REFERENCES TO RELATED PATENTS

3,577,946 May 1971 Horton 3,919,957 November 1975 Ray et al. 3,982,492 September 1976 Steddum 4,436,050 March 1984 Liden 4,498,142 February 1985 Liden 4,844,659 July 1989 Hunter et al. 5,147,148 September 1992 White et al. 5,330,293 July 1994 White et al. 5,421,676 June 1995 Wybro et al. 5,964,550 October 1999 Blandford et al. 6,190,089 February 2001 Bennett, Jr. et al. 6,447,208 September 2002 Huang et al.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

No Federal sponsorship, research support or assistance of any kind was provided, obtained or utilized in the development of the invention and technology described herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Floating production facilities for deepwater oil and gas production include floating production and storage/offloading vessels (FPSO), semi-submersibles, spars and tension leg platforms (TLP). Each facility has inherent advantages and limitations in water depth, ability to host processing facilities, cost and means of fabrication and installation.

2. Description of the Related Art

Existing Tension Leg Platforms (TLP) consist of two principal types of semi-submersible like platforms (conventional or extended base) and several smaller types of mini-TLPs. The semi-submersible type TLPs consist of columns connected by pontoons and of tendons attached to the columns or to extensions that radiate outwards from the columns. Mini-TLPs consist of a central floating column and tendon moorings that are attached to the ends of structural arms that radiate outwards from the central column. Each TLP type is moored to the ocean floor using steel tubulars.

There are some structural inefficiencies and design and operating limits inherent in each existing TLP design type. For instance, existing technology TLPs design the column as being continuous and the pontoons as being intermittent. This results in high stress loads arising in way of the column and pontoon connections.

Difficulty with existing TLP designs arises in way of the tendon porch connections to the TLP. In a conventional TLP, the tendon porches are connected directly to the columns, which are invariably circular. This conventional attachment requires complex fabrication in order to mitigate stress concentrations, achieve fatigue limits and allow for construction. Extended base TLPs and mini-TLPs with their extensions also tend to have high stress concentrations requiring additional analysis, design and construction in order to achieve safety and fatigue limits. These shortcomings are difficult to increase fabrication complexity and associated cost.

Another problem that arises with existing TLP technologies pertains to locating the tendon porches as far apart as possible in order to maximize mooring spread. On existing TLPs in order to maximize the mooring spread one invariably maximizes the inter-column spacing which results in greater topsides steel and higher deadweights, and therefore reduced payloads. Maximizing the mooring spread for mini-TLPs requires longer tendon arms and extensions which bring about additional structural and stability problems for this type of platform.

Existing TLPs must balance the weight (tension) of the mooring system and payload against the buoyancy provided by the hull (pontoons and columns, extensions too, where applicable). Existing TLP designs are limited by water depth in which they can be economically installed and operate: As water depth increases, tendon length and size, and thus weight (deadweight) increases quickly and a functional limit of water depth (decreasing payload) is soon realized. Mini-TLPs reach their functional limit at shallower water depths, because of their single columns provide even less buoyancy to offset deadweight and payload.

SUMMARY OF THE INVENTION

This specification describes a Column Offset Tension Leg Platform (COTLP), a new type of Tension Leg Platform (TLP). A COTLP is a tension leg platform that is arranged in a fractal, spirally symmetric pattern and that consists of continuous structural pontoons onto which columns are mounted and tendons are attached. There are several advantages to a COTLP. A COTLP affords significant design and construction advantages over existing TLP types. The COTLP incorporates better hydrodynamic, mooring, systems integration and payload characteristics. A COTLP advances technology to operate safely in water depths exceeding current TLP limits. As a consequence, a COTLP will be more cost effective to design, build, install and maintain and a COTLP can be installed in greater water depths than existing TLPs.

DESCRIPTION OF THE DRAWINGS

Drawing #1 (Dwg #1) Illustrates key dimensions of the COTLP.

Drawing #2 (Dwg #2) illustrates the key parameters of the pontoon and column repeated structural element.

Drawing #3 (Dwg #3) illustrates how the repeated structural elements of the CTOLP can be arranged in a fractal pattern to form a complete floating hull form COTLP.

Drawing #4 (Dwg. #4) illustrates the important structural system difference between a COTLP and existing TLP technology.

DETAILED DESCRIPTION OF THE INVENTION

A COTLP uses similar, repeated structural elements to from fractal like symmetric arrangements. The structural elements consist of a continuous structural pontoon on to which a column is mounted and supported. The pontoon can also be circular or polygonal in cross. The column can be circular or any regular polygon in plan. Tendons are used to moor the platform in position. The tendons are connected to the pontoons and may consist of any number of tendons required. The column is offset from the tendon connection end of the pontoon along the longitudinal axis of the pontoon. For a column offset from the pontoon end, Cx, the offset of the edge of the column from the end of the pontoon along the longitudinal axis of the pontoon, is given by the following:

0.05 PL<=Cx<0.5 PL−3/4 (CD or LC)   (Eq. 1; See Dwg #2)

Where:

PL=the pontoon length

CD=column diameter, in the case of round columns

CL=column length, in the case of a square or regular polygonal column; if the column is rectangular or with one side longer than another, then CL=the length of the longest side.

The column may also be narrower or wider than the width of the pontoon on which the column rests. However, the column width, CW should be within the following limits.

0.9 PW<=CW<=1.2 PW   (Eq. 2; See Dwg #2)

Where:

PW=pontoon width in the case of pontoons with square or rectangular pontoon sections; =the pontoon diameter in the case of pontoons with circular cross sections.

By combining Fractal COTLP elements, that is combining one column and one pontoon pairing (element), with another or several other column and pontoon parings (elements), one can develop several different fractal, symmetric arrangements. The fractal design of the COTLP allows for the use of multiple numbers of pontoons and columns including three (3), four (4), five (5) and six (6) pontoon and column element arrangements to support deck structure and platform facilities. The advantage of the CTOLP number of column arrangements allows for design and installation of the COTLP in waters deeper than current technology allows by providing for additional mooring points, increased buoyancy and reducing hydrodynamic response to waves. In addition, the spacing and arrangement/configuration flexibility of the COTLP means that inter-column spans can be shortened thereby reducing deadweight, while at the same time increasing buoyancy.

By using continuous pontoons, the pontoons can be designed and fabricated with through structural members that provide greater structural efficiency and support for the columns. The intrinsic design characteristic of the perpendicular arrangement of the column to pontoon connection means that critical structural connections can be more easily designed and fabricated, and subsequently, when the platform is in service, these same connections can be more easily inspected and maintained (if necessary). Furthermore, in a COTLP, by designing the pontoon as continuous, and landing the column on top of the pontoon, stress concentrations in way of the connection of the column to the pontoon are much reduced. In effect, in a COTLP the pontoon acts as a buoyant submersible hull on which the column is supported.

The intrinsic perpendicularity of the COTLP and the continuous pontoon structure also results in tendon connections to the pontoons boundaries and does not result in any undue stress concentrations arising from complicated structural connections at non perpendicular boundaries. A COTLP attaches tendon porches to the pontoon sides or to the pontoon end: Both connections are perpendicular and independent of the column.

The COTLP design also realizes perpendicular connections and penetrations for critical connections, systems and components. For example, the arrangement of the COTLP means that all penetrations from the column into the pontoon can be made perpendicular to the boundary between the column and pontoon. Furthermore, the continuity of the pontoon means that all penetrations along the length of the pontoon will also be perpendicular to the watertight boundaries. In the COTLP design, all penetrations are perpendicular to boundaries and do not, as in other TLP types, require any additional piping turns. Generally, perpendicular penetrations are easier to fabricate and install and are less likely to leak or fail than penetrations that go through boundaries at angels or that prefaced by additional piping turns. The COTLP captures this benefit and thereby facilitates construction and makes for a safer installation and operation.

Modular construction of the pontoon is also facilitated, because the COTLP is designed using similar, repeated structural and functional units that are assembled in fractal patterns. That is, a simple pattern repeated can develop a complex and sturdy structure.

Tendon distribution and mooring is also improved in a COTLP. A COTLP can provide greater spacing between tendons and subsea mooring than existing TLPs. This results in better mooring arrangements and holding power for COTLPs than for comparable existing TLPs.

Hydrostatically, the COTLP offers advantages in that continuous pontoons can support columns and tendon loads better than existing TLPs. Furthermore, a better hydrodynamic balance can be achieved because the COTLP can be designed with arranging the pontoon and column elements to compensate for and dissipate wave energy thereby reducing fatigue to tendons and hull structures. 

1. A tension leg platform structure for an offshore platform in which at least three buoyant, structurally continuous pontoons and structurally intermittent support columns are arranged in a fractal manner about a vertical, central axis to arrive at a semi-submersible, tendon moored floating platform.
 2. The substructure of claim 1 such that buoyant pontoons are continuous structural members.
 3. The substructure of claim 1 such that the connections between buoyant columns and pontoons is such that only one column is structurally integral to one pontoon.
 4. The substructure of claim 1 that the connections of the tendons to the pontoons does not result in a radial mooring arrangement and platform symmetry, but rather a spiral mooring geometry and spiral platform symmetry.
 5. The substructure of claim 3 such that the columns rest upon or are connected on top of the pontoons.
 6. The substructure of claims 2 and 3 allow for more optimal structural design and construction technology/methodology.
 7. The substructure of claims 1 and 4 that the spiral arrangement allows for more than three (3) columns to be supported, in increments of one (1): that is, three (3) columns, four (4) columns, five (5) columns, and so forth.
 8. The substructure of claims 1 and 6 that the spiral arrangement allows for greater dispersion of the mooring foundations on the sea floor, which in turn provide mooring performance advantages.
 9. The substructure of claims 1 and 6 that the spiral mooring arrangement allows for of one (1), two (2), three (3), and so forth, as many tendons per pontoon as required. 